1. Field of the Invention
The present invention is directed to a method for determining the location of a positionable object in an examination subject by means of magnetic resonance, as well as to a magnetic resonance apparatus for implementing the method.
2. Description of the Prior Art
A comparatively good accessibility to the patient during the examination is established with a nuclear magnetic resonance tomography apparatus of the type marketed, for example, by Siemens AG under the name "Magnetom Open.RTM.". This apparatus allows working at the patient with interventional instruments during the MR imaging. Typical applications are, for example, surgery (particularly what is referred to as minimally invasive surgery) and biopsy, whereby the position of the instrument can be observed on a picture screen. Of course, chronologically and spatially exact information about the position of the instrument in the body are required for these procedures. Essentially two problems arise given such applications.
A real-time monitoring of the instrument position with the required spatial resolution and an adequately high signal-to-noise ratio makes extreme demands of the speed of the data acquisition and processing when entire datasets must be successively updated. There are various proposals for solving the speed problem. Thus, for example, German OS 43 17 028 discloses a method wherein, after the registration of an overall dataset, only a sub-region of the dataset that arose from the preceding measurement is updated using measured data of a following measurement. A central region of the k-space, which essentially defines the image impression, is thereby constantly updated; in addition, changing sub-regions of the overall dataset are updated. Measurement time is saved to a considerable extent because the entire dataset is not constantly updated, so that the chronological resolution of the method is correspondingly improved in view of the moving instrument.
German OS 195 28 436, corresponding to U.S. Pat. No. 5,687,725, discloses wavelet coding of the raw datasets, which have been frequency-coded in a conventional way in the direction of the motion path of an interventional instrument, with the wavelet coding being perpendicular to the motion path. Differing from conventional frequency coding with Fourier transformation, wavelet functions are spatially localized, i.e. wavelet functions are generated at different locations over the observation window. The region of the motion path thus can be designationally imaged.
A further problem is that instruments, probes, catheters, etc., which are introduced into the body to be examined, are not visible in the MR image without further measures. Protons are primarily employed for image generation. The instruments utilized in interventional applications, however, usually do not contain a concentration of protons adequate for the imaging, so that a direct visual supervision in the MR image is hardly possible. Given known arrangements, the location determination therefore ensues with the assistance of a radio-frequency coil attached to the interventional instrument that measures the nuclear magnetic excitation in its immediate environment. Typically, a dataset of the entire volume of interest of the examination subject is first registered and a reference image is acquired therefrom. An antenna that covers the entire examination region is connected to a radio-frequency reception unit. Subsequently, the radio-frequency reception unit is switched to the radio-frequency reception coil attached to the interventional object. The position of this radio-frequency reception coil, and thus of the interventional instrument, is determined by exciting the entire examination region and a gradient is subsequently activated in a first direction, so that the signal measured with the radio-frequency reception coil is unambiguously frequency-coded in this orientation. The frequency of the maximum amplitude is identified after a Fourier transformation of the registered data. The position of the radio-frequency reception coil in the direction of the first gradient can be unambiguously derived from this frequency. This sequence is then repeated with gradients in the two other spatial directions. The position of the reception coil in the examination region can be unambiguously determined after three such sequences.
The position of the radio-frequency reception coil acquired in this way can then be mixed into the previously acquired reference image dataset. To that end, the slice corresponding to this position is selected. The correct position is identified and marked within this slice.
Due to the limited pickup speed of the MR method given the currently available pulse sequences, the following problem occurs. It must be expected that the position of the patient will change over the time span of an interventional operation. The once-registered reference image dataset thus no longer coincides with the current conditions over the course of an examination. The reference dataset must be constantly updated in order to avoid having the position of the interventional instrument being incorrectly mixed into the reference image. Given conventional arrangements, this reference dataset must basically cover the entire volume in which the interventional instrument can be theoretically located. Since the reference dataset thus becomes extremely large, this procedure is very time-consuming, so that one rapidly encounters limits in the MR imaging with respect to the required chronological resolution in combination with an exact spatial resolution.